This invention relates generally to transdermal drug delivery systems, and more particularly, to a transdermal drug delivery composition wherein a blend of polymers is utilized to affect the rate of drug delivery from the composition. More specifically, a plurality of polymers having differing solubility parameters, preferably immiscible with each other, adjusts the solubility of the drug in a polymeric adhesive system formed by the blend and modulates the delivery of the drug from the composition and through the dermis.
The use of a transdermal composition, for example a pressure-sensitive adhesive containing a medicament, namely, a drug, as a means of controlling drug delivery through the skin at essentially a constant rate, is well known. Such known delivery systems involve incorporation of a medicament into a carrier such as a polymeric matrix and/or a pressure-sensitive adhesive formulation. The pressure-sensitive adhesive must adhere effectively to the skin and permit migration of the medicament from the carrier through the skin and into the bloodstream of the patient.
Many factors influence the design and performance of sustained or controlled release drug delivery products, and dermal delivery systems in general, including drug properties, optimum delivery rate, target site(s), type of therapy (short-term or chronic), patient compliance, etc. Among the drug properties which are known to influence the rate of release or permeation, or both, into the skin are the physicochemical properties, including molecular size, shape, and volume; solubility (both in the delivery system and through the skin); partitioning characteristics; degree of ionization; charge; and protein binding propensity.
When a drug is contained in a carrier, for example, a pressure-sensitive adhesive for transdermal delivery, the rate of administration may be affected by the rate of release of the drug from the carrier, as well as the rate of passage of the drug through the skin. These rates vary from drug-to-drug and from carrier-to-carrier. A variety of mathematical equations have been proposed in the prior art to describe theoretically the fundamentals of mass transfer phenomena involved in diffusion through a carrier and development of a flux across a membrane such as the skin.
Transdermal drug delivery systems can be divided into two general groups: system-controlled and skin-controlled devices. With skin-controlled devices, net drug delivery is controlled by the rate of drug permeation through the skin. Skin-controlled systems can be further subdivided into monolithic devices and reservoir devices.
Generally, a monolithic system comprises a drug dispersed or dissolved in a matrix comprising a homogeneous polymeric material of, illustratively, silicone adhesive, silicone rubber, acrylic adhesive, polyethylene, polyisobutylene, polyvinyl chloride, nylon, or the like. The drug is dissolved in the polymeric matrix until its saturation concentration is reached. Any additional drug, remains dispersed within the matrix. As drug is removed from the surface of the matrix, more of the drug diffuses out of the interior in response to the decreased concentration at the surface. The release rate is therefore not constant over time, but instead gradually decreases as the drug concentration decreases.
The flux, or percutaneous absorption rate of drugs through the skin, is described by Fick's first law of diffusion: EQU J=-D(dC.sub.m /dx),
where J is the flux in g/cm.sup.2 /sec, D is the diffusion coefficient of the drug through the skin in cm.sup.2 /sec, and dC.sub.m /dx is the concentration gradient of active agent across the skin.
In order to modify the rate of delivery from a monolithic transdermal device and into the dermis, the prior art has typically focused on selecting a specific single-polymer matrix or a blend of soluble (miscible) polymers. Illustrative examples are the novel polymers described in U.S. Pat. No. 4,898,920 and U.S. Pat. No. 4,751,087. There is a need in the art to modify the rate of delivery while using commercially available polymer components.
Another common technique for modifying the rate of drug delivery is the addition of a vehicle or enhancer to the formulation to increase the solubility of the drug within the polymer matrix, such as by adding a co-solvent such as polyhydric alcohols; and/or to change the skin permeability, such as by adding enhancers such as ethanol. There is a further need to be able to modulate the delivery of a drug from a polymer matrix without adding vehicles or enhancers.
There is no example in the prior art of using a simple blend of adhesive polymers to affect the rate of drug delivery from a monolithic adhesive-based transdermal composition. However, U.S. Pat. No. 4,814,168 granted Mar. 21, 1989 and a continuation-in-part thereof, U.S. Pat. No. 4,994,267 issued on Feb. 19, 1991, both assigned to Noven Pharmaceuticals, Inc., Miami, Fla., disclose the use of a multipolymer, specifically an ethylene/vinyl acetate copolymer or an ethylene/vinyl acetate/acrylic terpolymer, a rubber and a tackifier in a carrier composition to improve the adhesive properties. The composition of U.S. Pat. No. 4,994,267 further includes an acrylate polymer in the system for additional improvement to the adhesive properties.
Drug concentration in a monolithic transdermal delivery device can vary widely depending on the drug and polymers used. For example, certain drugs are effective in low doses and therefore the transdermal formulation may involve low concentrations, illustratively, 5% or less by weight of the medicament in an adhesive. Other drugs, such as nitroglycerin, require large doses to be effective and the transdermal formulation therefore, may involve high drug concentrations, approximately between 5 to 40% or more by weight in an adhesive. Low concentrations of medicament typically do not critically affect the adhesion, tack, and shear resistance properties of the adhesive. However, low drug concentrations in the adhesive can result in difficulties in achieving an acceptable delivery rate of the medicament. High concentrations, on the other hand, frequently affect the adhesion properties of the adhesives. The deleterious effects are particularly exacerbated by drugs which also act as plasticizers, or solvents, for the polymeric adhesive material (e.g., nitroglycerin in polyacrylates).
There is a need in the art for an adhesive composition for transdermal delivery systems which can selectably incorporate low concentrations of drug and deliver same at an adequate and controlled rate or incorporate high concentrations of drugs while retaining good physical adhesive properties.
It is, therefore, an object of this invention to provide a transdermal drug delivery system wherein the rate of drug delivery from the transdermal composition may be selectably modulated.
It is another object of this invention to provide a transdermal drug delivery system wherein the rate of drug delivery from the transdermal composition may be selectably modulated by adjusting the solubility and/or diffusivity of the drug in the multiple polymer adhesive system.
It is also an object of this invention to provide a transdermal drug delivery system wherein the multiple polymer adhesive system is simple to manufacture.
It is a further object of this invention to provide a transdermal drug delivery system wherein drug-loading of a multiple polymer adhesive system may be selectably varied without adverse effects on drug delivery rate and adhesive properties, such as adhesion, tack, and shear resistance.
It is additionally an object of this invention to provide a transdermal drug delivery system wherein a novel multiple polymer adhesive system is provided which has desirable physical properties.